1. Field of the Invention
The present invention relates to an actuating device employed in a steering system for a vehicle, and particularly, to an actuating device for changing a gear ratio employed in a steering system for a vehicle, which can be constructed to solve the problem of misalignment of a planetary gear shaft, compensate the tolerance of a sun gear, and minimize vibration and noise.
2. Description of Related Art
A steering apparatus is provided for changing the driving direction of a vehicle according to a driver's intention, and is an auxiliary apparatus that can change the center of rotation of front wheels of the vehicle to enable the vehicle to be driven in the direction desired by a driver.
Meanwhile, a power steering apparatus utilizes a device that provides auxiliary steering power when a driver operates a steering wheel, to supplement the operating force applied to the steering wheel by the driver so as to enable the driving direction of the vehicle to be easily changed with little force.
The power steering apparatus is mainly classified into a hydraulic power steering apparatus (HPS) and an electric power steering apparatus (EPS).
The hydraulic power steering apparatus is an apparatus in which, when working oil is supplied from a hydraulic pump, connected to the rotating shaft of an engine, to an operating cylinder, a rack bar is horizontally moved by means of auxiliary steering power generated by the pressure of the working oil to rotate both wheels. The above apparatus enables the driver to operate the steering wheel with little force.
In contrast, the electric power steering apparatus (EPS) is an apparatus in which, instead of a hydraulic pump and an operating cylinder, a rack bar or a column comprises a motor and an electronic control unit (hereinafter, referred to as an “ECU”) to allow auxiliary steering power to be supplied by driving the motor.
Recently, to actively cope with the effect of the external environment, such as strong side winds or the left-right asymmetrical state of roads, on vehicles that are being driven, an active front wheel steering apparatus has been developed and marketed. In the active front wheel steering apparatus, a steering shaft between a steering wheel and a steering column is separately constructed, the steering angle generated by the steering wheel, which is operated by the driver, is not directly transmitted to the steering column, the steering angle, which frequently varies, is continuously sensed, the sensed steering angle is compared/analyzed together with all kinds of information on the vehicle, and an electric signal is then transmitted to a motor, which controls the rotation of a steering shaft of the steering column, to adjust the steering angle.
Further, this active front wheel steering system is provided with an actuating device for changing the gear ratio between an input and an output of a gearing device.
FIG. 1 is a view showing the structure of one example of a conventional active front wheel steering system for a vehicle, and FIG. 2 is a schematic view showing an actuating device for changing the gear ratio.
As shown in FIG. 1, a conventional active front wheel steering system comprises a steering wheel 10, a steering shaft 13 connected to the steering wheel 10, a steering column 15 encompassing the steering shaft 13, an input angle sensor 17 provided at the lower end of the steering column 15, an actuating device 19 provided below the input angle sensor 17 to change the gear ratio, a first motor 11 for operating the actuating device 19, a first motor positioning sensor 12 provided on the first motor 11, an output angle sensor 14 provided at the lower portion of the actuating device 19, a vehicle speed sensor 16 for sensing the vehicle speed, a rack-and-pinion mechanism 21 provided under the actuating device 19 for converting rotational movement of the steering shaft 13 into a linear movement of the rack bar 27, a second motor 18 for supplying auxiliary steering power to the rack bar 27, and an electronic control unit (ECU) 25 for receiving various signals from the input angle sensor 17, the first motor positioning sensor 12 and the output angle sensor 14.
As shown in FIG. 2, the conventional actuating device comprises an input shaft 31, a first sun gear 22 provided at the lower end of the input shaft 31, an input-side planetary gear 37 externally engaged with the first sun gear 22, an output-side planetary gear 38 connected to the input-side planetary gear 37 through a planetary gear axis 39, a second sun gear 23 externally engaged with the output-side planetary gear 38, an output shaft 24 connected to the second sun gear 23, a worm wheel 35, and a worm shaft 36.
The actuation principles of the conventional active front wheel steering system and the actuating device for changing the gear ratio having the above structure are as described below.
When the driver operates the steering wheel 10 to rotate the input shaft 31, the first sun gear 22 provided at the lower end of the input shaft 31 is rotated in response to the rotation of the input shaft 31.
In this case, in addition, the input-side planetary gear 37, which is externally engaged with the first sun gear 22, is rotated, and the output-side planetary gear 38, which is connected to the input-side planetary gear 37 via the planetary gear axis 39, is rotated.
Finally, the second sun gear 23, which is externally engaged with the output-side planetary gear 38, is rotated, and the output shaft 24, which extends from the second sun gear 23, is rotated in response to the rotation of the second sun gear. As a result, steering force generated from the steering wheel 10 operated by the driver is transmitted to the rack-and-pinion mechanism 21.
However, the above process is performed when the first motor 11 is not driven. If the first motor 11 is driven, the following change occurs.
If the vehicle speed sensor 16, for sensing the vehicle speed, the input angle sensor 17 and the output angle sensor 14, for sensing any change in the steering angle, and the first motor positioning sensor 12 generate electrical signals, respectively, and these electrical signals are transmitted to the ECU 25, the ECU 25 transmits an electrical signal to the first motor 11 to drive the first motor 11.
Also, as the first motor 11 is driven by the above process, the worm shaft 36, which is connected to a motor shaft (not shown), is rotated, and the worm wheel 35, which is engaged with the worm shaft 36, is rotated according to the rotation of the worm shaft. Finally, a carrier 29, which is formed integrally with the worm wheel 35, is rotated to enable the ratio of the output angle with respect to the input angle to be adjusted.
If the vehicle is driven at high speed, in the active front wheel steering system, when the driver turns the steering wheel 10 in one direction to rotate the input shaft 31, the first sun gear 22 and the input-side planetary gear 37, the ECU 25 controls the first motor 11 to allow the worm wheel 35 as well as the carrier 29 to be rotated in the direction by which the rotation of the output-side planetary gear 38 connected to the input-side planetary gear 37 is prevented, and so the rotation angle of the output shaft 24 eventually becomes smaller than that of the input shaft 31, to assist in the safe driving of the driver.
On the other hand, if the vehicle is driven at low speed for parking and so on, in the active front wheel steering system, the ECU controls the first motor 11 to allow the rotation angle of the output shaft 24 to become larger than that of the input shaft 31, and so the driver can easily drive the vehicle with little force.
Additionally, in the conventional actuating device shown in FIG. 2, the first sun gear 22, the second sun gear 23, the input-side planetary gear 37 and the output-side planetary gear 38 are generally formed in a helical gear shape. The helical gear is a gear in which a row of gear teeth is inclined with respect to, rather than parallel with, the axis of rotation of the gear. As compared with a gear in which the row of gear teeth is parallel with the rotation axis of the gear, since the length of the contact portion of a gear tooth engaged with a gear tooth of another helical gear is longer, the helical gear can transmit more power, and the helical gear rotates more smoothly due to the inclined row of gear teeth.
In the conventional actuating device constructed as above, however, since both end portions of the planetary gear shaft 39 are supported by bearings (not shown), the axis of the planetary gear shaft 39 becomes misaligned due to an error in the location of the bearings disposed at both ends of the planetary gear shaft 39. Additionally, for this reason, friction is increased, to thus generate noise.
Additionally, in the conventional actuating device, a damping structure for axially supporting the planetary gear shaft 39 is not applied. Thus, the conventional actuating device is disadvantageous in that the tolerance of the gear is not compensated, and vibration and noise are generated because no axial damping effect can be obtained.
Furthermore, in the conventional actuating device, since the worm wheel 35 and the carrier 29 are formed in a barrel shape and the bearings should be installed at both end portions of the planetary gear shaft 39, the weight, machinability of parts, and assemblability are deteriorated.